The objective of this study is to determine the role played by light-induced free radicals in chemically induced skin photosensitivity. Free radicals have been detected during light irradiation of 4-aminobenzene-sulfonamide (I), 4-nitrobenzenesulfonamide, 4-nitroaniline, 4-aminobenzoic acid (II) and 4-nitrobenzoic acid. This finding may help to explain the phototoxic and photoallergic properties of I and II.